assert((len("abc") == 3))
assert((substr("abcd",0,2) == "ab"))
assert((index_of("aaa bbb aaa bbb","bbb") == 4))
assert((last_index_of("aaa bbb aaa bbb","bbb") == 12))
assert((char_at("致远电子",0) == "致"))
assert((char_at("致远电子",1) == "远"))
assert((char_at("致远电子",2) == "电"))
assert((char_at("致远电子",3) == "子"))
assert((char_at("致远电子",-1) == "子"))
assert((char_at("致远电子",-2) == "电"))
assert((char_at("abc",-2) == "b"))
